My 2013 328 has bad rattles with the Harman Kardon(HK) turned on from mid to high volume. The dealer was amazed how bad the car was rattling. They claim that all f30 models have these rattles.

I have the 600W hk system in my F30. Guess what? NOT A SINGLE RATTLE when I play all types of music softly, loudly and anywhere in between.

Tell your dealer to stop making generalised comments as they have not seen every F30. They haven't seen mine.

It's called cause & effect. Something is causing the rattling. Being so evident, it should be a fairly straightforward fix. Insist they hire an audio professional to check it out if they cannot. OR take your car to another dealer (if convenient) & have someone else check it out.